The most abundant antibody isotype in the serum is IgG, which has a critical role in mediating protection against pathogens as well as in mediating allergic and inflammatory responses that hasten recruitment of immune system components to the tissues, mucosae, and dermal surfaces. Junghans, Immunologic Research 16(1):29 (1997). Moreover, IgG is also a key component of a variety of autoimmune diseases.
Under normal conditions, the half-life of IgG in the serum is a prolonged period relative to the serum half-life of other plasma proteins. For example, the serum half-life of IgG is 5 to 7 days in mice and 22-23 days in humans. Roopenian et al., J. Immunology 170:3528 (2003); Junghans and Anderson, Proc. Natl. Acad. Sci. USA 93:5512 (1996). In part, this long half-life of IgG is due to its binding to the Fc receptor, FcRn. FcRn binds to the constant region of IgG, known as Fc. Although FcRn was originally characterized as a neonatal transport receptor for maternal IgG, it also functions in adults to protect IgG from degradation. FcRn binds to pinocytosed IgG to protect it from degradative lysosomes and then recycles it back to the extracellular compartment. Junghans and Anderson, Proc. Natl. Acad. Sci. USA 93:5512 (1996), Roopenian et al., J. Immunology 170:3528 (2003). If the concentration of IgG reaches a level that exceeds available FcRn, unbound IgG will not be protected from degradative mechanisms and will consequently have a shorter serum half-life. Brambell et al., Nature 203:1352 (1964). Furthermore, although FcRn is expressed on the cell surface, it is believed that much of FcRn is intracellular and is associated with endoplasmic vesicle membranes, and that the interaction between IgG and FcRn occurs intracellularly after IgG is pinocytosed into the cell.
Structurally, FcRn exists as a heterodimer composed of one light chain, termed a beta (β) chain, and one non-covalently bound heavy chain, termed an alpha (α) chain. The light chain of FcRn, which is better known as β2-microglobulin (β2m), is also a component of the Major Histocompatibility complex I (MHC I). The FcRn a chain is a 46 kD protein composed of an extracellular domain that is divided into three subdomains, α1, α2, and α3; a transmembrane region; and a relatively short cytoplasmic tail. Burmeister et al., Nature 372:336 (1994).
FcRn was first identified in the neonatal rat gut where it functions to mediate the absorption of IgG antibody from the mother's milk and facilitates its transport to the circulatory system. Leach et al., J. Immunology 157:3317 (1996). FcRn has also been isolated from the human placenta where it mediates absorption and transport of maternal IgG to the fetal circulation. In adults, FcRn is expressed in epithelial tissue (U.S. Pat. Nos. 6,030,613 and 6,086,875), such as, but not limited to, the lung (Israel et al., Immunology 92:69 (1997)), intestinal and renal proximal tubular epithelium (Kobayashi et al., Am. J. Physiol. (2002); Renal Physiol. 282:F358 (2002)), as well as nasal, vaginal, and biliary tree surfaces. In addition, the ubiquitous expression of FcRn on endothelial cells is suggestive of its importance in IgG homeostasis. Ward et al., International Immunology 15(2):187 (2002); Ghetie et al., Eur. J. Immunology 26:690 (1996).
In general, FcRn functions in IgG homeostasis by antagonizing the catabolism of IgG by binding to the Fc portion of IgG. Once pinocytosed, IgG is captured in intracellular vacuoles that are beginning to fuse with acidic early endosomes. Crystallographic studies suggest the stoichiometry of the FcRn-IgG complex is composed of two molecules of FcRn to one IgG (Burmeister et al., Nature 372:336 (1994)) and binding of the two molecules is thought to occur on the Fc portion of IgG near the interface of the CH2 and CH3 domains (Burmeister et al., Nature 372:379 (1994)). The endosomal fusion event represents one stage of the lysosomal degradative pathway, which degrades or catabolizes complex biomolecules contained within the endosome into constitutive components. The low pH environment of the early endosome promotes the binding of FcRn to IgG as well as the release of any antigen bound to the IgG. Consequently, the antigen is degraded and the FcRn-IgG complex avoids degradation and is ultimately recycled to the cell surface where the physiological pH of the extracellular environment promotes the release of the IgG from FcRn.
In order to study the contributions of FcRn to IgG homeostasis, mice have been engineered so that at least part of the genes encoding β2m and FcRn heavy chain have been “knocked out” so that these proteins are not expressed. WO 02/43658; Junghans and Anderson, Proc. Natl. Acad. Sci. USA 93:5512 (1996). In both of these knockout mouse lines, the half-life and the concentration of IgG in the serum are dramatically reduced, suggesting a FcRn-dependent mechanism related to IgG homeostasis.
The inhibition of IgG binding to FcRn reduces IgG serum half-life by preventing IgG recycling. Therefore, agents that block or antagonize the binding of IgG to FcRn may be used in methods of regulating, treating or preventing disorders involving immune reactions, such as, e.g., autoimmune and inflammatory diseases and disorders characterized by the presence of inappropriately expressed IgG antibodies. One example of a method of blocking IgG Fc binding to FcRn involves the generation of blocking antibodies to FcRn. Indeed, antibodies capable of blocking the binding of FcRn with IgG have been generated using a FcRn heavy chain knockout mouse line (WO 02/43658). Recently, peptides have been identified that bind to FcRn complexes. Kolonin et al., Proc. Natl. Acad. Sci. USA 99(20):13055-60 (2002); U.S. Pat. No. 6,212,022. However, at this time additional agents are needed to regulate, treat, or prevent conditions, diseases, and disorders characterized by immune reactions.